President Dmitry Medvedev has submitted amendments to the State Duma, Russia's lower house of parliament, introducing chemical castration for criminals guilty of sexual offences against children, a Duma official said on Tuesday.

The castration bill, designed to "increase the effectiveness of the prevention of the strikingly brutal crimes" against children, also introduces life sentence as a possible punishment for pedophiles who are repeat offenders.

The bill also stipulates voluntary medical treatment, including chemical castration, for other rapists.

Medvedev, who first proposed the introduction of chemical castration for pedophiles in May, said punishment for such criminals should be "as harsh as possible." "A liberal approach here is totally unacceptable," he said.

Medvedev suggested that castration of convicted pedophiles should remain voluntary, while the United Russia party, which holds a majority in parliament, has insisted it should be obligatory.

Chemical castration of pedophiles is used in several countries such as Canada, Britain, France, Germany, Denmark, Israel, Norway, Sweden, Poland, as well as in some U.S. states, and consists of a series of regular chemical injections that block the effects of the male hormone testosterone.

In early June, the State Duma passed amendments in the second reading tightening punishment for sexual crimes against children, including child pornography and increasing the term after which a jailed pedophile can apply for early release.

The third reading was reportedly delayed, however, after Medvedev prepared his own amendments.

In April, the head of Russia's Investigative Committee announced that it had prepared a bill introducing chemical castration for pedophiles. More than 9,500 sexual crimes against children were committed in Russia in 2009, including more than 960 rapes, according to the committee.

Medvedev highlighted the importance of measures to protect children in his annual address to the nation in November 2010, saying that protecting children was paramount for the future of Russian society.